Although it can be applied to any desired reinforced foam materials, the present invention and the problems on which it is based are explained with respect to sheet-like sandwich structures with a foam core that is reinforced by fibers infiltrated with a resin, enclosed between outer layers.
Composite materials, and in particular so-called sandwich structures, have a wide range of applications in the area of aircraft construction on account of their good ratio of stiffness and strength to density. Sandwich structures generally have an upper outer layer and a lower outer layer, between which there is, for example, a core structure of vertically running cells of hexagonal cross section, to increase the stiffness.
An alternative to such honeycomb structures are rigid foams. They have advantages over honeycomb structures, inter alia in the area of thermal and acoustic insulation and in the process technology for the production of sandwich structures. One disadvantage of these foam structures is the lower mechanical strength obtained with the same density.
To compensate for this lower strength, sandwich structures with foam cores can be additionally reinforced by introducing bracing elements into the core. Sewing methods in particular offer the possibility of introducing fibers and filaments to form struts at different angles and with a density varying over the component. The technically achievable stitching speeds allow rapid reinforcement of the component to be possible. After a resin infiltration process, the pierced regions contribute to the mechanical reinforcement of the basic foam material. Such structures are already used in sandwich structures in the construction of trucks and in shipbuilding.
DE 10 2005 024 408 A1 discloses a sewing method in which a through-hole is first pierced into the foam material from one side with the needle, to then allow the needle to be used to pick up a bundle of fibers provided on the other side and draw it into the foam material. Since the bundle of fibers largely fills the through-hole, only little resin is incorporated in a subsequent infiltration, which leads to an improved ratio of the mechanical strength achieved to the density of the sandwich structure and makes corresponding structures suitable for use in aircraft construction.
The reinforcing effect is in this case based on tensile/compressive loading of the struts introduced. These can only absorb forces when they are connected at both ends to a pressure-resistant element, generally an outer layer of the sandwich structure. If they are severed in the middle or detached at one end, they are mechanically ineffectual.
Since the struts typically run at different defined angles in relation to the outer layer, there is the problem that, in the case of localized working at a working location of a sandwich structure reinforced in this way, the mechanical strength of the sandwich structure is reduced in a region that is increased beyond the worked region, in which struts lose their mechanical effect. In this increased region, the reinforced foam material becomes the strength-determining element, which has the effect that the mechanical properties greatly change locally, in particular in the case of lightweight foams.